EIGRP

Mental model

EIGRP is the protocol that takes the best of both routing-protocol families and avoids their worst sins.

• From distance-vector (RIP-style): simple, no link-state database, just trust your neighbors’ summaries.
• From link-state (OSPF-style): fast convergence, no routing loops, smart algorithm.

The killer feature: DUAL (Diffusing Update Algorithm) keeps a backup path ready in the routing table. When the primary fails, EIGRP doesn’t have to flood updates and re-compute — it instantly switches to the backup. Sub-second convergence is normal.

EIGRP was Cisco-only for two decades. Cisco opened it in 2013 (RFC 7868). It’s still mostly seen in Cisco-only environments — multi-vendor networks default to OSPF.

The metric

EIGRP’s metric is a formula combining up to five values weighted by K-constants. In practice only two matter:

metric ≈ (10^7 / minimum-bandwidth + total-delay) × 256

• Bandwidth — the slowest link in the path, in Kbps
• Delay — sum of interface delays, in tens of microseconds

Don’t touch the K-values. The K1–K5 constants control which factors are weighted. Defaults are K1=K3=1, everything else 0 — bandwidth + delay. Mismatched K-values between neighbors prevents adjacency formation. Just leave them alone.

Successor vs Feasible Successor (THE concept)

EIGRP terms that confuse everyone:

• Successor — the primary next-hop for a destination. The route that goes in the routing table.
• Feasible Successor (FS) — a backup next-hop, pre-validated as loop-free.

For a backup path to qualify as a Feasible Successor, it must satisfy the Feasibility Condition (FC):

The neighbor’s reported distance to the destination must be LESS than my distance to the destination via the successor.

In English: the backup neighbor must already be closer to the destination than I am (via my primary path). If yes, taking that route can’t cause a loop, so it’s safe to pre-install as backup.

When the successor dies, EIGRP doesn’t have to ask anyone — it knows the FS is loop-free and starts using it immediately.

Commands

Basic config (classic, with network statements)

R1(config)# router eigrp 100
R1(config-router)# eigrp router-id 1.1.1.1
R1(config-router)# network 10.0.12.0 0.0.0.3
R1(config-router)# network 10.0.13.0 0.0.0.3
R1(config-router)# network 192.168.1.0 0.0.0.255
R1(config-router)# passive-interface default
R1(config-router)# no passive-interface GigabitEthernet0/0

AS number 100 must match on every router for them to form neighbors. Unlike OSPF process IDs, this matters.

Named-mode config (the modern way)

R1(config)# router eigrp CORP
R1(config-router)# address-family ipv4 unicast autonomous-system 100
R1(config-router-af)# eigrp router-id 1.1.1.1
R1(config-router-af)# network 10.0.0.0 0.255.255.255
R1(config-router-af)# af-interface default
R1(config-router-af-interface)# passive-interface
R1(config-router-af)# af-interface GigabitEthernet0/0
R1(config-router-af-interface)# no passive-interface

Named mode is more readable and required for some advanced features. Either form works for CCNA.

Verification

R1# show ip eigrp neighbors
R1# show ip eigrp topology
R1# show ip eigrp interfaces
R1# show ip route eigrp

• show ip eigrp neighbors — your bidirectional partners. Should show your peers in up state.
• show ip eigrp topology — successors and feasible successors. If you see FD/RD columns, you’re looking at the heart of DUAL.
• show ip route eigrp — routes EIGRP installed in the routing table.

Common mistakes

1. Mismatched AS numbers. Two routers running EIGRP on the same wire with different AS numbers won’t form a neighborship. Easy mistake, silent failure. Check show ip eigrp neighbors.

2. Wildcard mask backwards. Same trap as ACLs and OSPF — wildcard is the inverse of subnet mask. For a /24 it’s 0.0.0.255, not 255.255.255.0.

3. Touching K-values. Don’t. They must match between neighbors. The defaults are correct for 99% of cases.

4. Forgetting passive-interface default. Like OSPF, EIGRP tries to form neighbors on every interface. A user-facing port is a security risk. Always default to passive, then explicitly enable peering.

5. Using auto-summary on classless networks. EIGRP used to auto-summarize at classful boundaries (10.0.0.0/8 etc.). This is wrong on modern networks. no auto-summary is the default on IOS 15+, but verify on older gear.

6. Setting feasible successor expectations too high. Not every destination has an FS — it depends on the topology. Without an FS, EIGRP falls back to the slower diffusing-update process (called “going active”) when the successor fails.

Lab to try tonight

1. Three routers in a triangle. Each with a loopback (1.1.1.1, 2.2.2.2, 3.3.3.3) and /30 links between them.
2. Configure EIGRP AS 100 on all three. Hardcode router IDs. Set passive-interface default, then enable only the WAN-facing interfaces.
3. Run show ip eigrp neighbors on each. Two neighbors per router.
4. Run show ip eigrp topology for the loopback of one router. Identify successor and feasible successor (if any).
5. Bring down a link between two routers. Watch show ip route eigrp re-converge — typically sub-second.
6. Bonus: increase a link’s delay with delay <tens-of-microsec> on the interface. Watch the metric change and the path selection adjust.

Cheat strip

Concept	Plain English
DUAL	Diffusing Update Algorithm — pre-computes backup paths
AS number	Must match on all routers for neighborship
Successor	Primary next-hop, in the routing table
Feasible Successor (FS)	Pre-validated backup, available for instant failover
Feasibility Condition	Backup’s reported distance < my distance via primary
Hello / Hold timers	Default 5s / 15s on LAN, 60s / 180s on slow WAN
Metric	Bandwidth + delay (default K-values). Leave K-values alone.
AD	90 (internal EIGRP), beats OSPF (110), RIP (120), but loses to static (1)